In general, encoders at the drive shaft, on the one hand, and additional vanes and sensors in the hoistway, on the other hand, are used to detect elevator car position. In an emergency affecting the power source, the position information can be written into an EEPROM or battery backed-up RAM to avoid the loss of that information. However, if the car moves independently of the elevator drive after the power supply has failed, or after getting the last position signal, actual car position is lost. In such a case, after connecting to the line voltage, the car position is usually obtained by means of an initialization run. U.S. Pat. No. 4,341,287 shows such a system. In other applications, multi-channel encoders are coupled to the car by a steel tape, holed or having magnets placed thereon, and the pulse train signals from the encoder are transformed to position information. The position initialization is accomplished by moving the car a few centimeters. Other prior art arrangements use coded indicia in the hoistway and appropriate readers of the indicia on the car or batteries for powering car position memory circuits during a power outage.
Absolute car position is the distance of the car from the top or bottom obtained without the need for a run to a terminal landing. Usually, when a car loses power, it loses position and has to run to terminal, whether at the top or bottom of the hoistway, to find where it is.
It would be desirable to determine absolute car position, without requiring the car to move to a predetermined hoistway and code readers on the car, and without requiring batteries or other auxiliary power supplies for storing the position the car had before power failure.
There is a wide range of requirements for elevator position indicators, but not every requirement must be fulfilled. For instance, in cases of an emergency, it is enough to know the approximate location of the car. In order to get the car position directly, the position sensor should be located in the hoistway. This requires a sensor system, which is insensitive to dust and acoustical interferences. For this reason, optical methods, such as infrared and laser, are unacceptable. Optical sensors are sensitive to dust because the light intensity decreases where a layer of dust appears on the lens or reflector. In addition, there is a need of regular maintenance which increases costs.